The present invention relates to the removal of foreign material from water before such water is discharged to its original source or to another location. In particular, the invention relates to the cleanup of hydrocarbons in water that is produced with crude oil before the produced water is discharged to a disposal area.
Attempts to meet discharge criteria by providing retention time with large tanks or flumes have not been successful. Discharge criteria are generally set by regulatory bodies, such as the Environmental Protection Agency ("EPA") or various state authorities. A widely-recognized standard, promulgated by the EPA for the Gulf of Mexico, allows a minimum of seventy-two (72) milligrams of oil per liter (mg/1, generally equivalent to parts per million, or "ppm") of the discharged fluid mixture, and requires a monthly average of not over 48 mg/1 (ppm). These standards are somewhat stringent in the context of general oil field production operations. To meet such criteria it is necessary to remove drops of oil on the order of 25 microns in diameter or smaller. Tanks and flumes with theoretical retention times large enough to accomplish this are subjected to severe short circuiting due to temperature and density difference. Previously, this has been overcome by the installation of several pieces of equipment each with its associated controls and shut-down sensors as required. These pieces of equipment include, for example, settling tanks, parallel plate interceptors, and gas flotation units.
In offshore fields the space available on an offshore producing platform is limited; thus, the area that is occupied by the several pieces of water cleanup equipment limits the amount of space available for other activities on the platform. In addition, the problem exists in many onshore oil fields where the ratio between the oil and water decreases with the age of the field, and thus the quantity of produced water that must be disposed of increases as the field ages. In the past the waters produced in these fields may have been disposed of in injection wells after being treated in large tanks, flumes, or settling ponds. With increasing water production these tanks are no longer capable of treating the water.